Changes related to the Model-View framework

The ImageFolderModel used for representing the images in "By Folder" filter now inherits KDirModel instead of fetching results from the database, while the rest of model fetches the data from the database created by the "koko" application itself.

There is a single AlbumView user interface for all different models. While applying filter just the model in the view changes. Whereas previously, different models ( ImageFolderModel, ImageTimeModel, ImageLocationModel) have different views